The local time of a semimartingale was originally studied by Levy as an occupation density. It also appears in Tanaka's formula, a one-dimensional generalisation of Ito's formula to the absolute value function. Various other extensions of Ito's formula for less smooth functions, including time-dependent extensions, have been established by different authors. We will look at those which include the local time. The problem of local time in higher dimensions, and recent progress, will also be discussed.